6,456 research outputs found
Vacuum phenomenology of the chiral partner of the nucleon in a linear sigma model with vector mesons
We investigate a linear sigma model with global chiral symmetry. The mesonic degrees of freedom are the standard scalar and
pseudoscalar mesons and the vector and axial-vector mesons. The baryonic
degrees of freedom are the nucleon, , and its chiral partner, , which
is usually identified with N(1535). The chiral partner is incorporated in the
so-called mirror assignment, where the nucleon mass is not solely generated by
the chiral condensate but also by a chirally invariant mass term, . The
presence of (axial-) vector fields modifies the expressions for the axial
coupling constants of the nucleon, , and its partner,
. Using experimental data for the decays and
, as well as lattice results for we infer
MeV, i.e., an appreciable amount of the nucleon mass originates
from sources other than the chiral condensate. We test our model by evaluating
the decay and the s-wave nucleon-pion scattering lengths
.Comment: 16 pages, 2 figures. To appear in Phys. Rev.
Accumulation horizons and period-adding in optically injected semiconductor lasers
We study the hierarchical structuring of islands of stable periodic
oscillations inside chaotic regions in phase diagrams of single-mode
semiconductor lasers with optical injection. Phase diagrams display remarkable
{\it accumulation horizons}: boundaries formed by the accumulation of infinite
cascades of self-similar islands of periodic solutions of ever-increasing
period. Each cascade follows a specific period-adding route. The riddling of
chaotic laser phases by such networks of periodic solutions may compromise
applications operating with chaotic signals such as e.g. secure communications.Comment: 4 pages, 4 figures, laser phase diagrams, to appear in Phys. Rev. E,
vol. 7
Periodic Neural Activity Induced by Network Complexity
We study a model for neural activity on the small-world topology of Watts and
Strogatz and on the scale-free topology of Barab\'asi and Albert. We find that
the topology of the network connections may spontaneously induce periodic
neural activity, contrasting with chaotic neural activities exhibited by
regular topologies. Periodic activity exists only for relatively small networks
and occurs with higher probability when the rewiring probability is larger. The
average length of the periods increases with the square root of the network
size.Comment: 4 pages, 5 figure
Coefficient of restitution for elastic disks
We calculate the coefficient of restitution, , starting from a
microscopic model of elastic disks. The theory is shown to agree with the
approach of Hertz in the quasistatic limit, but predicts inelastic collisions
for finite relative velocities of two approaching disks. The velocity
dependence of is calculated numerically for a wide range of
velocities. The coefficient of restitution furthermore depends on the elastic
constants of the material via Poisson's number. The elastic vibrations absorb
kinetic energy more effectively for materials with low values of the shear
modulus.Comment: 25 pages, 12 Postscript figures, LaTex2
Vacuum Properties of Mesons in a Linear Sigma Model with Vector Mesons and Global Chiral Invariance
We present a two-flavour linear sigma model with global chiral symmetry and
vector and axial-vector mesons. We calculate pion-pion scattering lengths and
the decay widths of scalar, vector, and axial-vector mesons. It is demonstrated
that vector and axial-vector meson degrees of freedom play an important role in
these low-energy processes and that a reasonable theoretical description
requires globally chirally invariant terms other than the vector meson mass
term. An important question for meson vacuum phenomenology is the quark content
of the physical scalar f0(600) and a0(980) mesons. We investigate this question
by assigning the quark-antiquark sigma and a0 states of our model with these
physical mesons. We show via a detailed comparison with experimental data that
this scenario can describe all vacuum properties studied here except for the
decay width of the sigma, which turns out to be too small. We also study the
alternative assignment f0(1370) and a0(1450) for the scalar mesons. In this
case the decay width agrees with the experimental value, but the pion-pion
scattering length is too small. This indicates the necessity to
extend our model by additional scalar degrees of freedom.Comment: 22 pages, 6 figure
The multilevel trigger system of the DIRAC experiment
The multilevel trigger system of the DIRAC experiment at CERN is presented.
It includes a fast first level trigger as well as various trigger processors to
select events with a pair of pions having a low relative momentum typical of
the physical process under study. One of these processors employs the drift
chamber data, another one is based on a neural network algorithm and the others
use various hit-map detector correlations. Two versions of the trigger system
used at different stages of the experiment are described. The complete system
reduces the event rate by a factor of 1000, with efficiency 95% of
detecting the events in the relative momentum range of interest.Comment: 21 pages, 11 figure
Stratified horizontal flow in vertically vibrated granular layers
A layer of granular material on a vertically vibrating sawtooth-shaped base
exhibits horizontal flow whose speed and direction depend on the parameters
specifying the system in a complex manner. Discrete-particle simulations reveal
that the induced flow rate varies with height within the granular layer and
oppositely directed flows can occur at different levels. The behavior of the
overall flow is readily understood once this novel feature is taken into
account.Comment: 4 pages, 6 figures, submitte
Chiral thermodynamics of dense hadronic matter
We discuss phases of hot and dense hadronic matter using chiral Lagrangians.
A two-flavored parity doublet model constrained by the nuclear matter ground
state predicts chiral symmetry restoration. The model thermodynamics is shown
within the mean field approximation. A field-theoretical constraint on possible
phases from the anomaly matching is also discussed.Comment: 8 pages, 2 figures, to appear in the proceedings of 6th International
Workshop on Critical Point and Onset of Deconfinement (CPOD), 23-29 August
2010 at Joint Institute for Nuclear Research, Dubna, Russi
The ATLAS Simulation: an LHC Challenge
The simulation program for the ATLAS experiment at CERN is currently in a full operational mode and integrated into the ATLAS common analysis framework, Athena. The OO approach, based on GEANT4, and in use during the DC2 data challenge has been interfaced within Athena and to GEANT4 using the LCG dictionaries and Python scripting. The robustness of the application was proved during the DC2 data challenge. The Python interface has added the flexibility, modularity and interactivity that the simulation tool requires in order to be able to provide a common implementation of different full ATLAS simulation setups, test beams and cosmic ray applications. Generation, simulation and digitization steps were exercised for performance and robustness tests. The comparison with real data has been possible in the context of the ATLAS Combined Test Beam (2004) and ongoing cosmic ray studies
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